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PEER
REVIEWED
52
FLUID THERAPY: PART 2
plasma. If the patient's sodium concentration
is >160 mEq/L, custom fluids can be made by
adding hypertonic saline to 0.9% saline to achieve
the desired sodium concentration ( BOX 2 ).
Free water can be provided enterally (voluntarily
or via a feeding tube) or by administering
5% dextrose in water. If 0.45% NaCl is
administered, half of the volume is free water;
the other half is equal to giving 0.9% NaCl.
During correction of FWD, the patient should also
receive a maintenance rate of a fluid that is close to
isotonic relative to its plasma. This may be 0.9% NaCl
if the patient is mildly to moderately hypernatremic.
If the patient is severely hypernatremic, a custom
solution may be required. Frequent (approximately
every 4 hours) monitoring of the sodium concentration
is necessary to ensure that it is not decreased too
quickly and to allow adjustments to the fluid therapy
plan. Ongoing hypotonic or free water losses may
require higher rates of free water be administered.
In patients that are hypernatremic because of sodium
gain, dilution of the extracellular fluid with free water
will cause further expansion of the intravascular
volume and may result in volume overload. Caution
should be used in patients that are oliguric or have
underlying cardiac disease. Frequent monitoring for
signs of volume overload ( BOX 3 ) is indicated.
In patients that have mild hypernatremia with
hypotonic fluid loss or decreased renal function
and a decreased ability to excrete a salt load,
0.45% NaCl can be used as a maintenance fluid
after volume depletion has been corrected.
Hyponatremia
Pathophysiology and Clinical Signs
Decreased serum sodium can occur in hypo-
osmolar, normo-osmolar, and hyperosmolar states.
Falsely decreased sodium can be seen in patients
with normal osmolality because another solute (eg,
glucose, mannitol) in the plasma is causing the
decrease in sodium or interfering with measurement
of sodium. TABLE 1 lists the effect of various
serum abnormalities on measured sodium. In the
case of hyperglycemia, the effect on measured
sodium is not linear, and the correction factor will
be higher with blood glucose >440 mg/dL.
1
True hyponatremia (with decreased osmolality) can
occur in conjunction with hypovolemia, hypervolemia,
or normovolemia. Hypovolemic hyponatremia can be
seen with gastrointestinal, third-space, or renal losses
and is frequently seen in hypoadrenocorticism.
1
BOX 2 Creating Custom Fluids for
Hypernatremic Patients
If a patient's sodium concentration is >160 mEq/L,
a fluid that is isotonic relative to the patient's
plasma can be made by adding hypertonic saline
(23.4% NaCl) to 0.9% saline.
The amount of NaCl in 23.4% saline is 4 mEq/mL.
The concentration of sodium in a liter bag
of 0.9% NaCl varies slightly; the average is
154 mEq/L.
It is helpful to measure the sodium concentration
of any customized solutions on a point-of-care
analyzer before administration.
For a patient with a serum sodium concentration
of 180 mEq/L:
Desired Na concentration = 180 mEq/L
0.9% NaCl = 154 mEq/L Na
180 mEq/L − 154 mEq/L = 26 mEq/L
26 mEq/L must be added to a 1-L bag of 0.9 NaCl
to match the patient's sodium concentration.
To obtain the amount of 23.4% NaCl that contains
26 mEq/L NaCl:
26 mEq/L/4 mEq/L = 6.5 mL
6.5 mL of 23.4% NaCl must be added to a 1-L bag
of 0.9% NaCl for this patient.
BOX 3 Clinical Signs of Fluid Overload
Increasing body weight
Increased skin turgor
Serous nasal discharge
Increased respiratory rate and effort
Peripheral edema
Body cavity effusions
Chemosis
Jugular venous distention